CHICAGO — Teachers in the nation’s third-largest school district were preparing to go on strike for the second time since 2012, as contract talks between the Chicago Teachers Union and financially troubled Chicago Public Schools approached a midnight deadline with no sign of an agreement.

The president of the teachers union said late Monday that the union was reviewing a new contract proposal from the school district, but it was too early to say whether it would be enough to avert a strike.

The union has directed its roughly 28,000 members to report to picket lines Tuesday morning unless they hear otherwise from union negotiators, who have said talks could continue until midnight.

All 652 schools will be open during normal school hours for the district’s 400,000 students if there is a strike, the district said.

The two sides held negotiations throughout the weekend. On Monday afternoon, teachers picked up strike placards and painted banners.

‘‘Nobody wants to go on strike but I think the teachers finally said ‘Enough is enough,’ ’’ said Nate Rasmussen, a preschool teacher at Beasley Elementary School on the city’s South Side.

Teachers have been without a contract since June 2015. The union wants no cuts to salary or benefits and an additional $200 million — or $500 per student — in spending to ensure adequate staffing and ‘‘to accommodate the needs of our children,’’ Union president Karen Lewis said Friday.

Associated Press